ABSTRACT Atrial fibrillation (AF) is a common arrhythmia that can have devastating consequences, including stroke and accelerated decline in cognitive function. Because AF is often asymptomatic and escapes clinical detection (subclinical AF), conventional methods for identifying AF have underestimated the population burden of AF. African Americans have a lower risk of clinically-recognized AF than whites, yet they suffer more stroke and cognitive impairment. A high prevalence of subclinical AF and of another important arrhythmia, supraventricular ectopy (SVE), may help to explain this paradox. In addition, several emerging risk factors for AF and SVE, including long-term changes in blood pressure, hypokalemia, the use of certain medications, and psychosocial factors, are modifiable and represent potential targets for AF prevention. The use of sensitive and unbiased methods to detect AF and SVE, and the discovery of novel modifiable risk factors for these arrhythmias, may offer opportunities for improved cardiovascular disease (CVD) prevention among African Americans. Our project will use a non-invasive electrocardiographic (ECG) monitoring device that is both well- tolerated and sensitive for detecting subclinical arrhythmia to measure subclinical AF, AF type (paroxysmal vs. persistent), and SVE frequency. In this ancillary study, we propose to conduct 14-day continuous ECG monitoring on 2,000 participants who return for Field Center Exam 4 of the Jackson Heart Study (JHS), a large prospective cohort study of CVD risk factors in African Americans. We will also use data from anticipated contract funded components of the Field Center Exam, including cognitive assessments and brain magnetic resonance imaging (MRI). Our goals are to assess the population burden of subclinical AF and SVE among African Americans, and to identify correlates of subclinical AF and SVE that may be modifiable risk factors or consequences of these arrhythmias. There are four aims. Aim 1: Estimate the age- and sex-specific prevalence of subclinical AF and AF type (paroxysmal vs. persistent) among African Americans in the JHS, and compare with the prevalence of subclinical AF and AF type among white participants undergoing 14-day continuous ECG monitoring in the Multi-Ethnic Study of Atherosclerosis and the Atherosclerosis Risk in Communities Study. Aim 2: Using data from JHS Exams 1-4, (a) evaluate whether traditional AF risk factors are associated with subclinical AF and SVE, and (b) identify novel, modifiable risk factors for these arrhythmias. Aim 3: Evaluate the utility of a clinical AF risk score, NT-proBNP levels, and risk factors identified from Aim 2 for the prediction of subclinical AF. Aim 4: In cross-sectional analyses, determine whether subclinical AF and SVE are associated with worse cognitive function and with brain MRI abnormalities. This project will generate new knowledge about subclinical AF and SVE that will be relevant to patients, and that will inform prevention and treatment strategies to reduce the burden of AF and AF-related complications among African Americans.